Problem: interrupted clone resume fails with index relation already exists error (#699)

The clone operation employs table copy workers to copy data. Once completed, it delegates index creation to index workers. If interrupted during index creation, resuming is impossible since the underlying table has already been copied.

Solution: On resume, let the table-data skip data copy if it is done but
still signal the index workers to proceed with creating indexes. Index
worker shall ignore if the index is already exists, else creates one.

Signed-off-by: Arunprasad Rajkumar <ar.arunprasad@gmail.com>